Closed Circuit Steam Cooling in Gas Turbines

Abstract

In the continuing effort to achieve better specific power and higher cycle efficiencies, gas turbine designers have through the years sought higher and higher firing temperatures. A large part of this gain in firing temperatures has been achieved through cooling the turbine nozzles and buckets. In almost all cases the coolant, usually air, is discharged into the gas path after performing its cooling function. This approach entails the double penalties of causing mixing losses and of producing a dilution of the hot gas stream by admixture of the lower temperature coolant. This paper presents a new cooling concept, developed under a study contract for Electric Power Research Institute, wherein high pressure steam is used as the coolant in a closed circuit steam cooling (CCSC) system. This not only avoids the mixing and dilution losses in the gas turbine, but permits recovery of the heat picked up in the coolant by expansion in a steam turbine. With CCSC, Brayton-combined cycle thermal efficiencies of 54% are projected using current materials and technology. With development of specific technologies, an ultimate efficiency for the Brayton-combined cycle of 57% is foreseen. This paper also discusses the sensitivity of the cycle performance to the design parameters. Performance of this CCSC cycle is compared to that of an advanced air-cooled Brayton combined cycle.